Multilayer PCB's are constructed by interleaving imaged conductive layers of copper with dielectric layers to make a multilayer sandwich. The dielectric layers are organic resin layers that bond the copper layers together. Typically, the layers of copper and dielectric are bonded together by the application of heat and pressure. The surface of the copper is smooth, however, and does not bond easily to the dielectric layer.
Improved bonding can be achieved by etching or otherwise roughening the surface of the copper to provide microscopic crevices and ridges in the surface of the copper. For example, mechanical means may be used to roughen the copper surface. Unfortunately, delicate circuit patterns are susceptible to damage if mechanically roughened. Thus, there is a need for a copper surface roughening process that does not require mechanical roughening of the copper surface.
Oxide processes are also known in which an oxide having a rough surface is formed on the copper surface. The oxide may be formed by chemical treatment of the copper. One such oxide process is described in U.S. Pat. No. 4,512,818, which provides a treatment solution for the formation of black oxide layers on copper surfaces of multi-layered printed circuits. The treatment solution comprises an oxidant and a hydroxide and is characterized by the addition of a water soluble or dispersible polymer to regulate the properties of the black oxide solution.
Another oxide process is described in U.S. Pat. No. 5,861,076. The '076 patent describes a bond enhancement process for promoting strong, stable adhesive bonds between surfaces of copper foil and adjacent resin impregnated substrates or superimposed metallic sublayers. According to the process of the invention, a black oxide-coated copper surface is treated with an aqueous reducing solution containing sodium metabisulfite and sodium sulfide to convert the black oxide coating to a roughened metallic copper coating. The roughened metallic copper-coated surface is then passivated and laminated to a resin impregnated substrate.
U.S. Pat. No. 5,492,595 also pertains to an oxide roughening process. The '595 patent describes a method for treating an oxidized surface of a copper film for bonding to a resinous layer. According to the method of the invention, an oxidized surface of a copper film having cupric oxide whiskers protruding therefrom is contacted with an acidic reducing solution containing thiosulfate to produce a reduced copper surface. The reduced copper surface is then rinsed with an acidic solution, and preferably treated with a passivating agent to minimize any reoxidation prior to laminate formation. A preferred passivating agent is 2-mercaptobenzothiazole.
Oxide processes, while well known, have many shortcomings. A typical oxide process is run at such high temperatures that the substrate is often distorted, leading to quality control problems and additional production costs. The oxidation process is also associated with uniformity problems in which portions of the copper surface are not oxidized or coated by the oxidizing solution. Uniformity problems lead to partial delamination in the multilayer PCB's. To avoid this problem the PCB is often run through multiple passes to obtain a more uniform oxide coating. Performing multiple passes adds considerably to production cost. Thus, there is a need for a copper roughening process that does not require multiple passes or high temperature, and that does not suffer from the uniformity problems of conventional oxide processes.
Another shortcoming of the typical chemical oxide modification process is that a strong reducing agent, such as dimethylamine borane, is applied to the oxide coating to obtain an even oxide coating. This type of adhesion promotion process produces an oxide coating that is fragile and prone to scratching during handling. Inspection of the circuitry prior to lamination is difficult because of the fragility of the oxide coating. Therefore, there is a need for an adhesion promotion process that permits a less problematic inspection after the adhesion promotion process and prior to the lamination step.
In response to the various problems associated with traditional oxide processes, and in particular their time consuming nature and high processing temperatures, alternative oxide coating processes have been developed. These alternative processes combine the oxidation function of the traditional processes with a controlled etch that actually roughens the underlying copper surface while oxidizing it at the same time. These alternative oxide coating processes tend to be much faster than traditional oxide processes because they form bonds with increased strength and therefore do not require multiple passes. In addition, the alternative methods do not require high temperature processing.
One alternative oxide coating process is described in U.S. Pat. No. 5,800,859. The process includes a treating step in which a metal surface is contacted with an adhesion promotion material. The adhesion promotion material includes 0.1 to 20% by weight hydrogen peroxide, an inorganic acid, an organic corrosion inhibitor and a surfactant. The surfactant is preferably a cationic surfactant, usually an amine surfactant and most preferably a quaternary ammonium surfactant.
Another alternative oxide coating process is described in U.S. patent application Ser. No. 09/479,089, which is incorporated herein by reference in its entirety. The '089 application describes a method and composition for providing roughened copper surfaces suitable for subsequent multilayer lamination. The method involves contacting a smooth copper surface with an adhesion promoting composition which includes an oxidizer, a pH adjuster, a topography modifier, and either a coating promoter or a uniformity enhancer.
While alternative oxide processes such as that described the '089 application are advantageous over conventional oxide coating processes for a variety of reasons, the roughened copper surfaces formed by such processes exhibit chemical sensitivity and thus tend to be susceptible to chemical attack. Chemical attack typically occurs during post lamination processing steps. After a multilayer copper and dielectric sandwich is formed through the lamination process, certain post lamination processing steps are performed to prepare the multilayer PCB.
For example, “through-holes” are drilled through the multilayer sandwich in order to connect the inner layers of the circuit board. The act of drilling these holes typically leaves traces of resin smear on the through-hole interconnections that must be removed by a desmear process. One desmear process involves the application of a solvent sweller and a permanganate etch which can chemically attack the bond between the copper surface and dielectric resin at the site of the through holes. The permanganate etch is typically followed by an acid neutralizer which can also chemically attack the bond and cause delamination. While other through-hole cleaning techniques are known, such as plasma etch or laser ablation, these processes generate intense heat which can also attack the copper/resin interface.
Once the desmear process is completed, the drilled holes are made conductive through direct metalization or similar processes. These processes involve numerous alkaline and acid processing steps, all of which can chemically attack the copper/resin interface. Further, the conductive through-hole is usually sealed with a layer of electrolytic copper. The electrolytic process involves alkaline and acidic baths which can also lead to chemical attack of the through-hole interconnects. The result of these chemical attacks is the delamination of the sandwich layers in the area of the through holes.
The chemically attacked area is termed “pink ring” or “wedge void” in the circuit board industry. The formation of pink rings or wedge voids represents serious defects in the PCB's, especially in an era when increasingly high quality and reliability are demanded in the PCB industry. Thus, there is a need for an improved alternative oxide coating process that provides a surface that is less susceptible to chemical attack during post-lamination processing steps.